Passive termination methods for SCSI (Small Computer Systems Interface) have traditionally provided reliable operation at low data transfer rates. At higher data rates, however, transmission line effects become troublesome and passive terminators do not adequately compensate for impedance mismatches. Voltage reflections due to impedance mismatch between the terminator and the line are detrimental to the overall noise immunity of the system and become evermore troublesome the faster data rates become. Also, since signal line and other parasitic capacitances become more significant at higher data rates, a passive terminator is ineffective at reducing the overall rise time of a signal negation since it cannot source sufficient current.
Further drawbacks of the passive terminator deal with low noise immunity and excessive power dissipation. Since the passive terminator Thevenin voltage is unregulated the passive terminator achieves a low noise immunity which impinges upon the terminator output specifications called out by the SCSI standard. Finally, the passive terminator continuously dissipates wasted power since there always exists a DC path from the bias supply to ground.
The Boulay terminator utilizes an active voltage regulator to maintain a constant voltage at one terminal of a 110 .OMEGA. resistor (the other terminal couples to the line). Because the Thevenin voltage is regulated, the output current is essentially immune to bias supply variations. Consequently, increased noise immunity is achieved as compared to the passive terminator. The average power dissipation of the Boulay terminator is also reduced since the only power dissipated in the Boulay terminator is the current required to power the regulator.
Although the Boulay terminator improves the noise immunity and power dissipation attributable to the terminator it does not address the timing issues associated with the finite rise time of a signal negation resulting from line and parasitic capacitances. If the rise time is long compared to the clock rate and there exists ringing on the transient edge because of impedance mismatches, erroneous sampling may result. A circuit for providing a higher source current is needed to drive the line from low to high faster while reducing ringing during such transitions.